Coaxial cable networks have been used to deliver high quality video programming to subscribers for many years. Conventionally, these networks have been unidirectional, broadcast networks with a limited number of channels and a limited variety of content provided to the subscribers. In recent years, cable companies have developed systems to provide bi-directional communication over their existing networks with a wider variety of services and content to their subscribers. For example, many cable companies now provide connection to the Internet through the use of cable modems.
The cable industry has developed a number of standards for delivering data over their networks to provide a uniform basis for the design and development of the equipment necessary to support these services. For example, a consortium of cable companies developed the Data Over Cable Service Interface Specifications (DOCSIS) standard. The DOCSIS standard specifies the necessary interfaces to allow for transparent, bi-directional transfer of Internet Protocol (IP) traffic between a cable head end and customer equipment over a cable or hybrid fiber/coax network.
A cable modem termination system (CMTS) is included in the head end of the cable network for processing the upstream and downstream transmission of data. In the upstream, the CMTS down converts the data signals to base band or a low intermediate frequency, then demodulates the signals. One problem with the design of the CMTS in many systems is in the complexity and expense of the down conversion circuitry. Typically, this down conversion is accomplished with a large number of analog components that impose requirements in space, expense, complexity, and time to implement and tune properly. To address some of these problems, digital down conversion circuits can be introduced in cable modem termination systems (CMTSs). Unfortunately, the use of digital down conversion circuits introduces substantial design complication. Specifically, when a digital down conversion circuit is used, it must interact with other circuits in the CMTS, including circuits on other chips and from other manufacturers. Each chip or circuit may have one or more independent time domains with various clock signals. Differences in the time domains introduced by these additional clock signals complicate the timing of the system.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for techniques for compensating for differences in clock signals from circuits that work together in a common system.